With well-developed horns and spikes protruding from the base of its domed skull and from its snout, Stygimoloch-its name refers to the Hell Creek site in Montana, where it was found-was more elaborately ornamented than most other Pachycephalosaurs. What we know about this relatively elusive plant-eating dinosaur has been gleaned from only about five skull fragments and parts of the body skeleton. Like its larger cousin Pachycephalosaurus, with which it shared the late Cretaceous North American landscape, Stygimoloch walked upright and probably had small forelimbs and a long, stiff tail. Males may have used the horns at the base of the skull for locking heads with opponents in head-pushing contests for winning mates. The horns, which were not very strong, may, on the other hand, have been purely ornamental and employed only for courtship displays.

Because fossil remains of Stygimoloch are so scare, out understanding of this dinosaur is still very limited. Those parts of the skull that scientists have so far been able to study show that holes present in the rear of the skull roofs of many pachycephalosaurs-known as the "temporal fenestrae"-had closed up in Stygimoloch. This indicates that Stygimoloch's skull was more robust and suggets that this dinosaur, along with Pachycephalosaurus, was one of the more advanced of the pachycephalolosaurs.

Stygimoloch lived in a lowland habitat, where the principal predators would have been large theropods such as Tyrannosaurus, Albertosaurus, and Aublysodon. As well as being on the lookout for these, Stygimoloch would also have needed to be wary of a much smaller, though agile and rapacious, predator that probably hunted in large packs-Dromaeosaurus.